1. Field of the Invention
Wet-process phosphoric acid as produced supra by the fertilizer industry is a complicated chemical mixture involving the eleven major chemical elements of P.sub.2 O.sub.5, CaO, SO.sub.3, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, K.sub.2 O, Na.sub.2 O, MgO, SiO.sub.2, F, and H.sub.2 O. Thus, according to the phase system rule, at any given temperature and pressure, ten stable solid phases can exist together in equilibrium. In most commercial products, these possible solids do not present a problem at filter grade, 30 percent P.sub.2 O.sub.5 acid concentrations obtained by the calcium sulfate dihydrate process. A previous report (Frazier, Lehr, and Dillard, TVA Bulletin Y113, NFDC, Muscle Shoals, Alabama) shows that 30 percent P.sub.2 O.sub.5 acid is a sufficient solvent for the associated impurities, so that only CaSO.sub.4.2H.sub.2 O is a byproduct. The results show that (Na,K).sub.2 SiF.sub.6 is precipitated with the gypsum but is redissolved during the wash cycle and returned to the acid product. Also, the report shows that the other possible, stable precipitates at 30 percent P.sub.2 O.sub.5, will require significant compositional increases in impurity levels to effect their crystallization as solid phases. The primary reason for this lack of impurity precipitation is the high quantity of free water available for ionization of the acid hydrogen which acts as an effective solvent. In strong acids (40-55 percent P.sub.2 O.sub.5) produced by the calcium sulfate hemihydrate process or by concentration of the 30 percent acid, two major factors contribute to the precipitation of impurities. First, as concentrations near 45 percent P.sub.2 O.sub.5 are attained, solvent water and ionized hydrogen are reduced to a very low level. Secondly, the dissolved impurities are concentrated, along with the P.sub.2 O.sub.5, above the saturation levels required for the precipitation of several solid phases. In fact, data will be presented to demonstrate that every component in commercial 45-55 percent P.sub.2 O.sub.5 acid can be precipitated to a solid fraction. A lack of control of the impurity ratios in a 45 percent P.sub.2 O.sub.5 acid process can result in sufficient P.sub. 2 O.sub.5 retention in the filter cake that further processing of the cake will be necessary to recover these P.sub.2 O.sub.5 values. Thus, an understanding of the unique chemical properties of these solutions is necessary in order to obtain high-analysis acid products without excessive P.sub.2 O.sub.5 losses. Data from our chemical study will show the compositional variations necessary to cause undesirable components to be precipitated and, at the same time, minimize the precipitation of P.sub.2 O.sub.5.
2. Description of the Prior Art
In the fertilizer industry, purified acids are necessary for the production of stable fluid fertilizers in order to avoid chemical and physical problems due to post precipitation of the dissolved impurities. Feed grade phosphate materials and phosphate detergents have even higher impurity restrictions than fertilizers. The prior art discloses many single-step processes for reducing specific impurities in wet-process acid products without considering interaction and catalytic effects of other impurities. Many of these, such as U.S. Pat. Nos. 3,935,298 and 3,562,769, recommend a pretreatment of the phosphate rock to prevent the incorporation of other impurities which reduce the effectiveness of their teachings. Others, such as U.S. Pat. Nos. 3,408,162, 3,554,694, 3,642,439, and 2,954,287, teach the use of agents which again singly reduce one impurity without restricting the levels of other impurities. The interaction effects of other materials are not considered; for example, U.S. Pat. No. 4,299,804 does not recognize the need for aluminum in reducing the magnesium content by adding fluoride even though the precipitating compound is a magnesium fluororaluminate. Thus, these teachings are restricted to acid products of high aluminum content. The present invention discloses the types and quantities of impurity precipitates that can be crystallized from strong (40-55 percent P.sub.2 O.sub.5) phosphoric acid solutions and the importance of the catalytic effect of ferric iron on maximizing the quantity of each. Likewise, the necessary amendments required for maximizing the quantity of each precipitate are presented.